Convert first contentful paint logging to the new UKM client API

components/ukm/ukm_entry.h

Index: components/ukm/ukm_entry.h
diff --git a/components/ukm/ukm_entry.h b/components/ukm/ukm_entry.h
index 28b6d7a7f300ae6245f59d1b1686ccca179f1022..8cac67bab1b477c5d7d66fc39772440cbe868be7 100644
--- a/components/ukm/ukm_entry.h
+++ b/components/ukm/ukm_entry.h
@@ -27,7 +27,9 @@ class UkmEntryBuilder;
 class UkmEntry {
  public:
   // Serializes the members of the class into the supplied proto.
-  void PopulateProto(Entry* proto_entry);
+  void PopulateProto(Entry* proto_entry) const;
+
+  const content::GlobalRequestID& source_id() const { return source_id_; }
 
   ~UkmEntry();